1. Field of the Invention
The present invention pertains to a coil tubing injection apparatus for inserting and removing a continuous length of tubing into a well.
2. Background
Oftentimes in the development and production of a well, an elongated tubing is inserted into the well from the surface for such purposes as the injection of certain types of fluids for stimulation of the production, displacing fluids in the well, and for performing cleaning operations on the production tubing. A continuous length of relatively thin walled steel tubing is inserted into the well from a large reel at the surface. The force required to inject or insert and to withdraw thousands of feet of such tubing are substantial.
U S. Pat. No. 4,585,061 illustrates a typical type of equipment used for tubing injection. A pair of opposed endless flexible conveyor members or chains are arranged to have elongated runs substantially parallel to each other and between which the tubing is straightened and propelled generally downward through a wellhead into the well proper. It is necessary to provide a substantial gripping force on the tubing to perform the injection and withdrawal operations. The tubing injection and withdrawal apparatus includes a frame which supports the opposed endless conveyor chains in such a way that the pull-down and hoisting forces may be applied to the tubing. The opposed endless flexible conveyor members are supported on the frame by spaced apart sprockets and engaged by means adapted to exert equalized gripping forces on the tubing along an elongated contact path between the conveyor members. Each of the endless conveyor members are made up of roller chains and associated support rollers which are engaged by elongated skates or ramp members having a plurality of segments which are, respectively, connected to separate opposed hydraulic cylinder actuators interconnected hydraulically in parallel and arranged mechanically to equalize and balance the clamping or gripping forces exerted on the tubing. The endless conveyor chains are also engaged with tensioning sprockets for maintaining the conveyor chains under a predetermined constant tension to eliminate slack in the chains. Chain tension is maintained by idler sprockets engaging each of the opposed gripping and conveying chains, which sprockets are connected to a pair of opposed linearally extensible hydraulic cylinder actuators which are mechanically and hydraulically connected to provide a uniform equalized tension adjustment force exerted on the endless conveyor chains. The drive mechanism for the gripping and conveying chains includes a pair of hydraulic motor and brake units mounted on a drive casing at the top of the support frame and are directly engaged with drive shafts for supporting and driving the chain driving sprockets. The opposed endless flexible conveyor members are disposed between plates on the frame and are trained over respective double idler sprockets rotatably supported by and between the plates. The conveyor members are each, respectively, drivably engaged with double drive sprockets. The drive sprockets are mounted on drive shafts which are drivenly connected to one of the motors. The conveyor members are each made up of a pair of spaced apart endless roller chains which are interconnected with each other and with a series of tubing gripper blocks by elongated cylindrical pins. The gripper blocks are configured such that the blocks may be nested one within the other. Each block is also provided with a support roller which is rotatably mounted on the pin. The gripper blocks are each provided with an arcuate recess having a radius of curvature only slightly larger than that required for conforming substantially to the radius of the curvature of the tubing. Accordingly, the blocks may be disposed in close fitting gripping relationship to the tubing along a linear path portion disposed between opposed parallel vertical runs of the conveyor members. In operation, the conveyor members forcibly grip the tubing passing along the axis such that the conveyor members grip the tubing so as to be substantially self-centering and load equalizing due to the hydraulic circuitry. The hydraulic cylinders are hydraulically connected in parallel so that the forces exerted by each cylinder on the opposed ramp members are substantially equal. Therefore, uniform loading on the tubing by the conveyor members along the runs may be accomplished.
Prior art tubing injection assemblies are complicated arrangements of support rollers and actuator members for applying a biasing force against the opposed conveyor chain courses or runs which are engaged with the tubing. One of the deficiencies of the prior art is that excessive gaps or cracks are required between adjacent blocks to allow the blocks to pass around the drive and idler sprockets. The prior art gripper blocks have a tang nested within a clevis. As the roller first passes over the tang and then the clevis, alternating forces are applied to the roller. Thus, these gaps or cracks cause an undesirable impact on the conveyor members as the rollers move over the cracks between adjacent blocks. The alternating nature of the gaps and thus the alternating forces causes axial bending in the outer shell of the rollers and eventual failure. It is also preferred to increase the bearing contact area between the rollers of the skates and the back surface of the conveyor members. A greater contact surface allows less pressure per square inch on the conveyor members giving the bearing surfaces of the rollers and bearings a longer life. Further, prior art hydraulic piston and cylinder assemblies are disposed on the ends of shafts which extend between the loops formed by the conveyor members. Such a location of the hydraulic assemblies increases the width of the injector mechanism. Further, the prior art apparatus applies a common gripping force along the length of the tubing so as to increase the stress and fatigue on that portion of the tubing in tension causing the tubing to have a shorter life. Prior art tubing injection devices also require two drive motors each weighing in excess of 640 pounds. These prior art motors are low speed, high torque hydraulic motors using approximately 3,000 pounds of pressure. It is desirable to reduce the weight on the frame caused by these large, heavy motors.
The aforementioned problems associated with the prior art apparatus are resolved by the present invention which provides an improved tubing injection apparatus.